Early remote control of electrical devices was accomplished through the use of radio frequency transmitters. Though these systems could address devices over a very large area, they were rather expensive, especially where several controllable devices were to be addressed. With advances in technology these transmitters have been reduced in size to pocket pagers and are still a common means of control, though their relatively high cost still limits their application. For less expensive control within smaller, more local regions, ultrasonic transmitters were used, as for home television sets, until these were largely replaced by infrared systems upon the advent of Light-Emitting Diodes.
Many systems exist where remote control is not required to be wireless. Utility companies within the power industry recognized that, with their vast array of long-line wired connections, wireless for them might not be necessary. In 1972 Paull (U.S. Pat. No. 3,656,112) taught one means of remotely reading utility meters. The power that supplied the circuitry to generate and transmit data when requested was drawn from the same power line that was being metered. However, Paull's application was limited to relatively local rather than long distance use since the transfer of data was still accomplished wirelessly by radio or acoustic signals,
Remote control systems that were fully wired, as opposed to being wireless, continued to use separate cables to carry power and data. For systems that covered a large area, this presented considerable expense, both in the cost of material for extra data cables and the labor involved in their installation. The application of control signals, data signals and power all being transmitted along a single pair of conductors was yet to come.
About the same time as Paull was using wireless means to read information from power lines, Durkee in U.S. Pat. No. 3,689,886 used sets of receivers attached to a power line to remotely control individual devices with both power and control carried by the same two conductors. Each receiver was tuned for sensitivity to one particular frequency from among a set of frequencies. A receiver would enable its controlled device if its assigned frequency was detected within a pre-assigned time slot following zero-crossings of the alternating current (AC) power. The delay and duration of the time slots were established by one-shots which were set manually. If the required frequency was not present within the prescribed window, the device controlled by the receiver would be disabled. This approach allowed for activation of only one device code at a time since the coded frequency had to be repeated on each cycle of the AC power in order to keep a device enabled.
To allow more than one device to be independently enabled at any given time, Sherwin described his “Frequency Selective Remote Control System” in U.S. Pat. No. 3,729,710 (1973). It used the two-wire power line to carry enabling signals to each of a set of remote devices. A set of discrete frequencies within the band of 10 kHz to 35 kHz were superimposed on the power line, each device's receiver being tuned to a specific frequency. By sequentially scanning multiple frequencies onto the power line, for say 10 msec each, more than one device could be activated within any particular period of time. Once enabled, a receiver would hold its controlled device in the enabled state long enough to allow the transmitter to scan all of the devices in the system before allowing the device to drop out for lack of freshly updated enabling input.
So far these were essentially analog systems where control capabilities were limited to two states, the controlled devices being either On or Off. Fowler (U.S. Pat. No. 4,093,946) in 1976 was among the first to describe a method that was more generally capable of communicating data over a two-wire power system. He taught how a current source could be used to superimpose pulses representing a binary signal onto a low impedance power line. However, the data carried by Fowler's system was limited to small numeric values. There was no encoding involved. Addresses of controlled devices and the data that they returned in response to their being selected was simply a count of pulses that occurred within a transmission window. The power supplied was DC (direct current) that was stored in a capacitor at each controller in order to carry it through what might be considered blackout periods during which the power line was shorted out in order to transmit data.
U.S. Pat. No. 4,200,862 assigned to Pico Electronics Limited of Fife, Scotland in 1980 by Campbell, et al. may have been the first to transmit control and data over a two-wire power line. The protocol described by Campbell, et al. has now become pervasive. Commonly known as the X-10 interface, a de facto standard, the home automation industry has grown up around this particular technology that was enhanced by Thompson's U.S. Pat. No. 4,628,440 and again by Campbell in U.S. Pat. No. 4,638,299 over the next few years.
Though the X-10 system allows selection of devices by binary codes up to 255, it is still somewhat limited in control and data capabilities. Initially X-10 controllers were limited to the On or Off states of previous designs with the additional capability of sending a “Dim” signal to lamps.
In the 1996 U.S. Pat. No. 5,491,463 issued to Sargeant, et al., mention is made of the allowance in the X-10 protocol for a reserved byte code to indicate an extension for transmission of a data stream. A special code was to be followed immediately by a series of 8-bit bytes with the first 8-bit byte identifying the number of data bytes to follow. However, at that time Sargeant, et al. were not aware of any commercially available product in which that feature had been implemented, and so they proceeded to describe how to do so.
Advances in the above-described technologies have continued, but with the emphasis being on automation within the home or other interior'spaces such as offices and hotels. The X-10 protocol with its use of 120 kHz signaling does not seem to have migrated well to the outdoors where the reactances and line lengths encountered are not suitable for binary data transmissions. Even the X-10's attempt to overcome noise by transmitting code patterns twice, first in their true form followed by a second copy in complementary form, has been insufficient to adapt the system for use in the less predictable outdoor and underground environments. Meanwhile, other techniques have developed for outdoor use, but there do not seem to be any standards, and no technique is nearly as pervasive as the X-10 is indoors.
Control of turf irrigation systems presents a frequent, if not the most common, requirement for outdoor automation. Several two-wire power and communications networks have been developed for underground turf irrigation systems. Their purpose is to reduce the amount of copper wire needed to install the system and to facilitate landscape upgrades and wiring repairs. Typically these have been developed with an emphasis on the communications means, many of them being wireless, and have overlooked the utility of using 24 Volts AC as the power option for the solenoid valves and other devices on the network. The departure from the commonly available, and therefore relatively inexpensive, 24 Volt AC components has necessitated the use of special valves operating at higher voltages or the need to generate a valve power signal that simulates 24 Volts AC, the latter alternative in some cases being at the expense of reliable valve response.
Some of the two-wire systems described above and their more recent enhancements have employed binary communications signals superimposed on the power network. Binary signals require that the two-wire bus be well-managed. That is, it may be required that the two-wire pair be twisted or that the conductors remain in close proximity to one another or that they must run from device to device with terminations and repeaters at each node. Without such control of line quality, reflections from the ends of the lines add signal artifacts, such as ‘porches’, to the ends of the binary signals. Poorly managed reactances cause distortion of digital signals in general, and the two-wire networks treated here are particularly susceptible.
A further limitation of two-wire systems based on the communication of binary signals is the result of noise spikes from the environment masquerading as data signals. Such broadband spikes easily disrupt the communication of binary signals leading to errors and the necessity for repeated transmissions at the expense of data rates. The disclosed invention deals with these issues from the basis of a scheme that combines Time Division Multiplex and Frequency Division Multiplex methods. The combination maintains the advantages of each method, achieving extremely low error rates while using simple hardware supported by sophisticated though relatively inexpensive microcontroller code.